1. Field of the Invention
The present invention relates to a light emission drive circuit for an organic electroluminescence element and a display device incorporating the drive circuit.
2. Description of the Related Art
Conventionally known is a display panel having organic electroluminescence elements (hereinafter simply referred to as organic EL elements), or one type of a capacitive light-emitting element, disposed in a matrix. An active display device designed to drive a display panel having organic EL elements has a light emission drive circuit configured for each pixel as shown in FIG. 1.
The light emission drive circuit for a single pixel shown in FIG. 1, has two FETs (Field Effect Transistors) 1, 2 and a capacitor 3 to drive an EL element 5. The FET 1 serves to write data and its gate G is connected to a scan line Yi to which a scan pulse is supplied, with the source S of the FET 1 being connected to a data line Xj to which a data signal is supplied. The drain D of the FET 1 is connected to the gate G of the FET 2 as well as to one terminal of the capacitor 3. The FET 2 serves to supply a drive current to the EL element 5 to drive the EL element 5, with its source S being connected to the other terminal of the capacitor 3 as well as to a common ground line 6. The drain D of the FET 2 is connected to the anode of the EL element 5, while the cathode of the EL element 5 is supplied with an output voltage Vee, as a negative potential from a power supply (not shown).
Now, the operation of the light emission drive circuit will be described below. First, when a scan pulse is supplied to the gate G of the FET 1 via the scan line Yi, the FET 1 turns on, allowing a current corresponding to the voltage of a data signal supplied via the data line Xj to the source S to flow from the source S to the drain D. During the ON period of the FET 1, the capacitor 3 is charged, and the charge voltage is supplied to the gate G of the FET 2. The FET 2 turns on (in an active state or in its saturation state) in response to the charge voltage. When the FET 2 is in the ON state, a forward voltage greater than or equal to a light emission threshold voltage is applied to the EL element 5 to pass a drive current from the ground line 6 through the source S—the drain D of the FET 2 and the EL element 5, thereby causing the EL element 5 to emit light. When the scan pulse is no longer supplied to the gate G of the FET 1, the FET 1 becomes an OFF state, and the FET 2 allows the charge stored in the capacitor 3 to hold the voltage of the gate G, and maintain the drive current as well as the light emission of the EL element 5 until the EL element 5 is scanned again.
As described above, in the light emission drive circuit for a conventional display device, as the FET 1 for writing a data signal via a data line onto a capacitor, the light emission drive circuit for a conventional display device employs a MOS-FET switching element of an organic semiconductor serving as a channel material. With such a MOS-FET switching element, it is necessary to scale up the MOS-FET itself in order to provide a current flowing therethrough in its ON state and sufficiently large enough to flow through a typical low-temperature polysilicon TFT (Thin Film Transistor) in a display panel. On the other hand, an increase in size of the MOS-FET would cause the parasitic capacitance between the gate and drain of the MOS-FET to increase accordingly. The presence of the gate-drain parasitic capacitance would cause an on-off control pulse signal voltage applied to the gate other than a drain-source ON current to be differentiated by the gate-drain parasitic capacitance into a charge/discharge current, which is in turn introduced into a data hold capacitor resulting in a change in the original capacitor hold voltage. This phenomenon is also found in a typical TFT of a polysilicon-based material. However, since its mobility of carriers of the organic semiconductor material is extremely lower than that of the typical polysilicon-based material, the drain-source current is relatively reduced to degrade the ratio between a current induced by the drain-source parasitic capacitance and the drain-source current. This causes the phenomenon to be evident to such an extent of interfering with the operation of the TFT formed of an organic semiconductor material. As a result, there was a problem that a voltage corresponding to a predetermined desired brightness was not applied to the gate of the FET 2, thereby causing a variation in the light emission brightness of the EL element 5.